1. Field of the Invention
This invention relates to an optical pickup tracking controller and an optical pickup tracking control method for positioning emission light on an information record track of an optical disc in an optical pickup for reading information from the optical disc based on return light from the optical disc.
2. Description of the Related Art
An optical pickup comprising a light source, a light transfer optical system, and a light detection system is used to read optical disc record information of music, data, etc., from return light provided by applying emission light from the light source to the information record side of the surface of an optical disc such as a CD (compact disc) or a CD-ROM (compact disc-read-only memory) and reflecting the applied light on the information record side of the optical disc.
To reliably read the information from the optical disc in the optical pickup, it is necessary to control so as to always apply emission light onto a record location on the information record side of the optical disc, such as an information record track, namely, it is necessary to perform tracking servo control.
A tracking servo control technique, for example, as shown in FIGS. 9(A) to 9(D) is known.
FIG. 9(D) is a sectional view of an optical disc wherein the surface of the optical disc is cut in the radial direction of the disc. A projection portion P is called a pit and a flat portion L is called a land. With the pit P, an information write state is shown and is read. If the optical pickup is moved in the radial direction of the disc (right and left in FIGS. 9(A) to 9(D)), emission light from the optical pickup moves so as to cross a pit string comprising pits P arranged in the tangential direction.
At this time, a reproduction (playback) RF signal from a photodetector receiving return light becomes as shown in FIG. 9(A). If the reproduction (playback) RF signal is made to pass through a low-pass filter (LPF) or the envelope of the signal is taken, a radial contrast signal RC as shown in FIG. 9(B) is provided. As shown here, the radial contrast signal RC has the center of the crest of the waveform matching the center position of the pit P and the center of the trough of the waveform matching the center position of the land L.
Light from the photodetector receiving return light is detected and is converted into an electric signal, whereby a signal TE as shown in FIG. 9(C) can be provided. The signal TE has a waveform passing through a zero level B (crossing zero) at the point corresponding to the center position of the land L as shown in the part of A.
Therefore, to perform tracking servo control, it is necessary to control the optical pickup so as to become the center of the trough of the radial contrast signal RC. This operation is called “bringing into tracking servo control”. In fact, tracking servo control is performed by performing feedback position control based on the TE signal so that the signal TE crosses zero as TE crosses zero at the center of the trough of RC. Although TE also crosses zero at the center of the crest of RC, the polarity of the differential value of TE is opposite at the position and if an attempt is made to perform feedback control, oscillation occurs. Therefore, it is necessary to perform feedback control based on TE at the center of the trough of RC; it is important to detect RC.
However, as optical discs with information recorded at a high density, such as a DVD (digital versatile disc), have been used in recent years, the following problem arises in the tracking servo control of the optical pickup in the related art described above:
As information is recorded on a disc at a higher density, the spacing between pits, namely, track pitch (the distance between the center positions in FIG. 9(D)) becomes narrower. Thus, the asperities of the waveform of the radial contrast signal RC become very small, the S/N ratio of the signal lowers, and the position of each trough of the radial contrast signal RC may be detected erroneously. If the position of the trough of the radial contrast signal RC is detected erroneously, bringing into tracking servo control is performed at the erroneous position and tracking control becomes unstable.